136
CHEMICAL ENGINEERING
pump. A piston, if newly packed may be perfectly tight, but will in time wear and exhibit signs of leakage. Depending upon the conditions of the valves and the speed the leakage may have any value between 0 and 100 per cent.
The hydraulic efficiency is the total head realized in percentage of the head produced in the pump. Eh = H -f- Hi, where H = total head pumped against (static head -f- friction + velocity head), and Hi = H + hydraulic losses in the pump. It gives information as to all losses due to the passing of the water through the pump, from the lower water level to the discharge nozzle. In a reciprocating pump where the velocity is low these losses are small and are difficult to determine. The hydraulic efficiency is, therefore, generally included in the term "mechanical efficiency" in which case Em - water horsepower -r- indicated horsepower or = water horsepower ~ brake horsepower.
Thermal efficiency, Et = 2,546 hp./S(H — A), where 2,546 = heat equivalent of 1 hp. in B.t.u. per hour; S = steam consumed, pounds per hour; H = total heat in 1 Ib. of steam at initial pressure; h = total heat in 1 Ib. of feed water.
PUMPING DIAGRAM, 100- TO 300-PT. HEAD (0. S. GALLONS)
The total efficiency is the product of the mechanical, the volumetric and the hydraulic efficiencies. E = Em X Ev X Eh.
Overall efficiency (in an electrically driven pump) E0 — water horsepower -~ electrical horsepower. It is the ratio of the actual power developed in the water end, i.e., pounds of water raised times equivalent head -f- 33,000, to the amount of electrical energy delivered to the motor as measured appropriately on the motor switchboard.
The water horsepower is the effective work done by the pump in lifting and transporting the water. It is equal to the pounds of water delivered per minute multiplied by the difference in elevation between the water level in the suction well and that in the reservoir plus the friction in the pipe divided by 33,000. HPW = WH ~ 33,000, where W — weight in pounds of water pumped = Gw, where G = amount of liquid actually discharged in United States gallons per minute, w — weight of 1 gal. (= 8.33 Ib. for water at 62°F.); H = total head in feet against which the pump operates (generally measured by a gage in the water column to which reading is added the vertical distance between the level in the section well and the center of the gage. 1 Ib. per square inch gage = 2.31 ft. head at 62°F.
The pump horsepower is the total work done by the pump, HPP = IIPW —